A variety of digital systems process pseudorandom bit sequences (PRBSs). In verifying the performance of these systems, it is often desirable to observe the waveform of the pseudorandom bit sequence at various locations within the system. Segments of the PRBS waveform can be reconstructed using sampled data acquisition systems, such as equivalent time sampling oscilloscopes.
Equivalent time sampling systems sample a waveform during repetitive cycles of the waveform. The timing of each new sample is progressively delayed relative to each of a succession of trigger events that are synchronous with the waveform. When a sufficient number of samples has been acquired, the sampled segment of the waveform is reconstructed and its characteristics are observed on a display or other output device. Equivalent time sampling systems described by Agoston in U.S. Pat. No. 4,812,769 and U.S. Pat. No. 4,678,345 incorporate oscillators to generate strobe signals that determine the timing of the waveform sampling. However, the oscillators add complexity to this type of sampling system.
Another known sampling system uses a startable oscillator that is started-up synchronous to a trigger event. Cycles of the oscillator's output signal are counted to generate strobe signals at predefined time delays relative to the trigger event. However, the frequency stability of the startable oscillator is compromised in order to achieve the quick start-up time, introducing uncertainty in the time delays. Frequency instability limits the accuracy of this type of sampling system, since the integrity of the waveform reconstruction depends on accurately controlling the time delay between the trigger event and the strobe signal.
Accordingly, there is a need for a sampling system that has low complexity and that accurately reconstructs repetitive waveforms, such as the waveforms of pseudorandom bit sequences or other digital signals present in digital systems.